Yes and no on the mechanic and alternator. Yes, in a way, especially in Service centers, they are trained to replace and nor repair. Outside mechanic would try to repair first before replace but then depends on what part again.
As for the alternator, olden type running on carbon which can be replaced. Nowadays many running on diodes and capacitors. I had my Honda one broke down many years ago, and when my mechanic open up, the problem was the diodes melted and is sealed, so no choice have to replace. In fact nowadays he say cannot repair unless send to those people who can recondition them, but also cannot last....
It's true that the rotor, diode pack and stator of an alternator typically aren't serviceable. But things like the copper brushes, bearings, rectifiers and pulley are. In many cases, the former group is the culprit. Reconditioned/Rebuilt alternators offer a cheaper alternative to getting a new one. But there's a risk here because we don't know the condition of the diode pack and stator. If we are lucky, the rebuilt alternator might last for many years.
A new unit is always better, or course. But it's expensive. I don't know what I'd do should this issue arise with my car: go for a rebuild or a new unit. I've read that an alternator's lifespan is unpredictable. Some cars, regardless of make, would need a new one after 60,000 km. But some others would go to 150,000 km and more. So it's something like a battery too, I suppose - various factors and variables including "luck" play a part.
I'm trying to be proactive by taking steps to prolong its service life. I'm driving a used car (14 years old) since three years ago so I don't know how long the alternator has been on it. Could be the original one which means it's likely at the end of its service life. Or maybe it's a newer unit (I hope so). Fortunately, diagnosing an alternator looks to be easier than with a battery. It would often give signs that you need to check it. Things like unusual noise coming from it, which could be the bearings or pulley. If not enough power is being generated, I believe a warning light should appear on the dashboard.
A multimeter would help provide a clearer picture. With the previous battery, when the engine is at idle, the multimeter showed 13.5V. All accessories switched on, 13.2V. These figures are "OK" but I think they are rather on the low side. With the engine running, it should be a bit above 14V with a very healthy alternator. These are signs that mine isn't at optimum level - only "satisfactory". I haven't checked with the new battery yet. Will do so after a week so that the new battery has been properly charged by the system through normal usage. I hope the reading will be 14V. If it still remains at 13.5V, I'll have to find a specialist to service the alternator the best that he can. It's better to service components "when we want to" rather than wait for something to happen first before we "are forced to":-) It could be at a very inconvenient place and time, and likely to be significantly more expensive than the preventive maintenance taken.